docs: def _build_cm(self, cm):
          """Convert the passed CM to the proper format, or construct the
          unitary CM if none was provided.
          """
          if cm is None:
              # Assume all are connected.
              cm = np.ones((self.size, self.size))
          else:
              cm = np.array(cm)
  
          utils.np_immutable(cm)
  
          return (cm, utils.np_hash(cm))